Use celery to replace our home grown background workers

This pull request replaces the query updater with Celery, to allow moving more logic to background jobs (like tracking events and in the future sending emails) and to simplify the code base.

The version was bumped to 0.4 to signify the change, as it will require changes from anyone currently running re:dash and updating to this version. Also a migration is required.

Another benefit from switching to Celery is the addition of actual queues, and the ability to give different data sources different number of workers. In most cases you won't need to deal with this, but it is useful with data sources like Redshift.